This invention relates to tomographic imaging, and more particularly to methods and apparatus for cone-tilted parallel sampling and reconstruction.
In at least one known CT imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
With the introduction of multi-slice CT imaging systems, research activities on multi-slice helical reconstruction have become the focus of many studies. Some known CT imaging systems address the issue of incomplete data sampling by using various approximation methods. These methods have been shown to be quite successful in producing clinically acceptable image quality even at fairly high helical pitches although little attention has been paid, however, in the area of reconstruction algorithms for the step-and-shoot (axial) mode CT imaging system.
For example, in some CT imaging systems, as the number of slices, and z coverage, increase, each projection view can cover a significant portion of an anatomical organ. As a result, the relative importance of the step-and-shoot or axial mode increases. For a 32-slice CT imaging system with a detector cell aperture of 0.625 mm, with a z-coverage of 20 mm, a cardiac scan can be completed in 6 axial scans. With a gantry speed of 0.5s per rotation, the entire heart can be covered in a single breath-hold. If a multi-sector reconstruction approach is utilized to improve temporal resolution and to freeze cardiac motion, the scan protocol is simpler as compared to the helical mode since the patient table moves to the next location only when sufficient data has been collected. In this approach, prospective gating of the x-ray with EKG signals can be easily achieved.
With the increased number of detector rows and increased z-coverage, cone beam artifact related issues become more important. For moderate cone angle, the Feldkamp (FDK) reconstruction algorithm has been shown to be sufficient. The FDK algorithm utilizes a three-dimensional backprojection to accurately backproject the filtered projections along the actual x-ray path, instead of the two-dimensional backprojection that approximates a ray path by a line that is parallel to the reconstructed image. Although the FDK algorithm has been shown to be sufficient for moderate cone angles, the FDK algorithm reduces the z-coverage of the scan.
In one embodiment, a method for reconstructing a computed tomographic (CT) image of an object is provided. The method includes initializing a CT imaging system in a step-and-shoot mode, scanning an object to generate a plurality of adjacent axial scans, wherein the distance between the adjacent axial scans is approximately equal to a projected detector height at the detector iso-center, and reconstructing an image of the object using the adjacent axial scans.
In another embodiment, a method for reconstructing a computed tomographic (CT) image of an object is provided. The method includes initializing a CT imaging system in a step-and-shoot mode, performing at least one axial scan to generate a plurality of projection samples, rebinning the projection samples to a set of tilted parallel geometry samples, and reconstructing an image of the object using the rebinned projection samples.
In a further embodiment, a computed tomographic (CT) imaging system for reconstructing an image of an object is provided. The imaging system includes a detector array, at least one radiation source, and a computer coupled to the detector array and the radiation source. The computer is configured to initialize a CT imaging system in a step-and-shoot mode, scan an object to generate a plurality of adjacent axial scans, wherein the distance between the adjacent axial scans is approximately equal to a projected detector height at the detector iso-center, and reconstruct an image of the object using the adjacent axial scans.
In a still further embodiment, a computer readable medium encoded with a program executable by a computer for reconstructing an image of an object is provided. The program is configured to instruct the computer to initialize a CT imaging system in a step-and-shoot mode, scan an object to generate a plurality of adjacent axial scans, wherein the distance between the adjacent axial scans is approximately equal to a projected detector height at the detector iso-center, and reconstruct an image of the object using the adjacent axial scans.